The present study represents a continuing effort to study structure-function relationships in cardiac muscle, including the atrium. The investigation involves the elucidation of the comparative ultrastructure of cardiac muscle performed on a variety of animals species. Special structures of the individual cardiac muscle cell are investigated in depth by morphologic and physiologic methods, each study complementing the other so that a better understanding of the function of the heart might evolve. In the present study emphasis is placed upon the question as to how the electrical impulses are conducted from one place in the heart to the other, and as to how the impulse reaching the individual cardiac cell is transformed into a contractile response, that is to say how excitation-contraction coupling takes place. Recent studies in our laboratory have raised important questions as to the function of certain compartments of the sarcoplasmic reticulum within the framework of excitation-contraction coupling and it is the main task of the current research to clarify the real function of the compartment in question. Within that same framework we are also investigating the overall function of the sarcoplasmic reticulum in different species of birds in a comparative sort of way since the structure of cardiac muscle cells in birds is different from that of mammals and some bird species are capable of extraordinary heart rates. We are using ultrastructural, biochemical and, more recently, physiology techniques to obtain the desired information.